PACKARD.] 



GEOLOGICAL SUCCESSION OF PHYLLOPODA. 



359 



species, differing decidedly from Estlieria morsel or Estherla mexioana. 

 It ratlier approaches E. jonesii from Cuba in the form of the shell and 

 style of marking- of the valves. It does not resemble closely any of the 

 fossil forms figured in Jones' Monograph of fossil Estheriie. The mark- 

 ings, however, i>resent some resemblances to E. middendorfii Jones, but 

 differs in the want of anastomosing cross-wrinkles between the ridges. 

 One valve and portions of others were x)reserved ; but none of them 

 show the breaks (umbones), though the form of the remainder of the 

 shell indicates that they were situated nearer the middle of the valve 

 than usual, i. e., between the middle and the anterior third of the shell. 

 The shell is deep, probably more so than in E. jonesii, though the valves 

 have evidently been flattened and somewhat distorted by pressure, but 

 a])X)arently the head-end was more truncated than iu E. jonesii, as the 

 edge of the shell and the parallel lines (or ridges) of growth along the 

 head-end are below bent at right angles to the lower edge of the shell. 

 The raised lines of growth are very numerous and near together ; they 

 are of nearly the same distance apart above near the beaks as on the 

 lower edge. The very numerous lines of growth are thrown up into 

 high sharp ridges, the edges of which are often rough, finely granulated, 

 and often the valleys between are rugose on the surface. In one or two 

 places a row of papillae for the insertion of S|)inules may be seen where 

 the shell has been well preserved, and between many of the lines of 

 growth there are irregular superficial ridges. Length 10™™; depth 



7 Rmni 



The valve is evidently that of an Estheria,much truncated anteriorly, 

 and with the lines of growth much thicker, higher, and closer together 

 than in any Korth American species known to us, and may prove, when 

 better specimens are found, to be allied to the Tertiary Siberian E. 

 middendorfii. 



The species is named in honor of the discoverer, J. W. .Dawson, LL. D., 

 who has so persistently and ably investigated the Leda clays of Canada. 



It should be observed that fig. 4 is not a particularly good representa- 

 tion of the fossil. 



A point of a good deal of interest in connection with this Quater- 

 nary species is that at present no species of Estheria is known to be 

 peculiar to the Atlantic province. Estheria mexicmia, however, ranges 

 as far east as Ohio, but this is not at all related to E. dawsoni. The 

 question arises where did the latter Quaternary species come from. It 

 is not an Arctic form, for no species of the genus is known to inhabit 

 the circumpolar region. It would seem as if it had been a Quaternary 

 survivor through the glacial period of a southern or Tertiary species. 



Geological Succession. — The following table gives a view of the geologi- 

 cal succession of the fossil Phyllopoda ; it is compiled from the works of 

 T. E. Jones and Gerstaecker, with the additions made, (1878-'81,) since 

 the publication of those works. 







Leda clay, Canada. 







Tertiary ? 







EoceneTertiary 



Sranchipus 



Fresh-water limestone, Isle of 



Wight.* 







Mesozoic? 







Wealden 













Oolite..-. 



Esttieria concentrica (Beau) 



Estheria murchisonice Jones 







Skve, Scotland. 









* Proceedings Geological Society of Loudon, 1878. Abstract in Aunals and Mag. Nat. Hist., 5th ser., 

 i, p. 99, 1878. 



